1. Field of the Invention
The present invention relates to a nonreciprocal circuit device such as an isolator used in a microwave band or the like and relates to a high-frequency circuit apparatus such as a communication apparatus provided therewith.
2. Description of the Related Art
Nonreciprocal circuit devices used in a microwave band or the like are disclosed in (1) U.S. Pat. No. 4,016,510, (2) Japanese Unexamined Patent Application Publication No. 52-134349, (3) Japanese Unexamined Patent Application Publication No. 58-3402, (4) Japanese Unexamined Patent Application Publication No. 9-232818, and (5) Japanese Unexamined Patent Application Publication No. 8 8612.
The above nonreciprocal circuit device is a component in which a ferrite plate is provided with center electrodes intersecting at a predetermined angle and then a static magnetic field is applied to the ferrite plate. By making use of a ferrimagnetic characteristic of the ferrite plate, the plane of polarization or a high frequency magnetic field caused by the center electrodes is rotated in accordance with Faraday""s law of rotation. This produces a nonreciprocal characteristic.
In the nonreciprocal circuit device such as the one in (5)-above that uses first to third center electrodes, the matching impedance of the third center electrode has a reactance component. Since the impedance depends on the frequency, the frequency range in which a preferable nonreciprocal characteristic can be obtained is narrow. That is, when the component is used as an isolator, the isolation characteristic inevitably has a narrow band.
The nonreciprocal circuit device that used two center electrodes has advantages of miniaturization and realization of a broader band. Further miniaturization of the nonreciprocal circuit device such as the isolator used in a communication apparatus has been also required in accordance with recent demands to miniaturize the communication apparatus in a wireless communication system.
However, when the size of a ferrite plate is greatly miniaturized to, for example, 0.5 mmxc3x970.5 mmxc3x970.3 mm while the conventional construction of the nonreciprocal component is maintained, as described below, since the length of the center electrode is shortened, the inductance component thereof is decreased. When the nonreciprocal circuit device is operated at a predetermined frequency, impedance matching cannot be obtained. Accordingly, the problem of increased insertion loss (IL) arises.
The circuit diagram of the conventional isolator is as shown in FIG. 8. When the inductance of center electrodes L1 and L2 impedance-match the capacitances of parallel capacitors C1 and C2, the impedance locus is the relationship as shown in FIG. 9. That is, when the impedance of the center electrode is a predetermined value, the impedance of the center electrode must be on a susceptance circle passing through 50 xcexa9 in order to connect the parallel capacitors so as to match the normalized impedance (50 xcexa9).
However, when the size of the isolator is desired to be approximately 3.5 mmxc3x973.5 mmxc3x971.5 mm or below, the size of the ferrite plate is 1.0 mmxc3x971.0 mmxc3x970.3 mm or below in a case in which it is a rectangular parallelepiped. In a construction such as that of the conventional isolator in which the center electrode is provided on only a principal-surface side of the ferrite plate, the inductance of the center electrode is decreased. Therefore, since the reactance is small at the operating frequency, the capacitances of the matching parallel capacitors must be increased. However, because of this, there arises a problem in that the operating frequency bandwidth is narrowed.
Furthermore, when a single-plate capacitor is used as the above matching parallel capacitors, the size thereof increases, which does not allow an isolator of a target size to be realized. For example, when it is intended to design an isolator with external dimensions of 3.5 mm square having an 800 MHz band, the capacitance of the parallel capacitor is required to be 6 pF for an inductance of the center electrode of 6.6 nH. Even though a high dielectric constant ceramic plate with a relative dielectric constant or, for example, 110 is used to form the matching parallel capacitors with a thickness of as thin as 0.17 mm, the dimensions of the capacitor are increased to as large as approximately 1.0 mmxc3x971.05 mm, which means that the capacitor cannot be contained in the isolator of the target size.
Overall miniaturization decreases the size or the center electrode, which decreases the inductance of the center electrode. When the inductance is too small to be on the susceptance circle passing through the normalized impedance (50 xcexa9), impedance matching cannot be obtained regardless of increased capacitance of the parallel capacitors. This increases the input/output impedances and worsens the insertion loss.
Objects of this invention are to provide a small nonreciprocal circuit device which exhibits a nonreciprocal characteristic over a wide band and which has low insertion loss and to provide a high-frequency circuit apparatus, such as a communication apparatus, using the nonreciprocal circuit device.
To this end, according to a first aspect of the present invention, there is provided a nonreciprocal circuit device including a first center electrode and a second center electrode intersecting each other, each having one end thereof grounded, a ferrimagnetic body provided in the vicinity of the first center electrode and the second center electrode, a magnet applying a magnetostatic field to the ferrimagnetic body, a series capacitor connected in series between the other end of the first center electrode and an input terminal and a series capacitor connected in series between the other end of the second center electrode and an output terminal, and a parallel capacitor connected in parallel between the other end of the first center electrode and a ground and a parallel capacitor connected in parallel between the other end of the second center electrode and the ground.
Since use of series capacitors and parallel capacitors enable input/output impedance to be positively matched, more insertion loss can be reduced, whereby miniaturization and a widened band can be achieved.
In the nonreciprocal circuit device, the first center electrode and the second center electrode may be wrapped around the ferrimagnetic body.
This enables the sufficient amount of inductance of the first and second center electrodes to be obtained even though a small ferrimagnetic body is used. Therefore, overall miniaturization can be achieved.
In the nonreciprocal circuit device, the intersection angle of the first center electrode and the second center electrode may be a predetermined angle in the range of 80 degrees to 100 degrees.
This enables low insertion loss and high non-reciprocal characteristic to be obtained.
In the nonreciprocal circuit device, the ferrimagnetic body may be polygonal plate.
This enables the magnetic coupling distance between the first and second center electrodes with respect to the ferrimagnetic body of the first and second center electrodes to be gained to be long. In addition, when the first and second center electrodes are wrapped around the ferrimagnetic body, wrapping is facilitated. Furthermore, even though the ferrimagnetic body is small, low insertion loss and high non-reciprocal characteristic can be obtained.
In the nonreciprocal circuit device, the magnet may be a rectangular parallelepiped.
This enables the intensity of the magnetostatic field applied to the ferrimagnetic body to be more increased in a limited volume in the nonreciprocal circuit device having an overall rectangular parallelepiped shape. Accordingly, low insertion loss and high non-reciprocal characteristic can be obtained. Furthermore, since the nonreciprocal circuit device can be constructed by cutting from a plate-like or rectangular parallelepiped magnetic material, manufacturing is facilitated.
Alternatively, in the nonreciprocal circuit device, the first center electrode, the second center electrode, the ferrimagnetic body, and the magnet are provided between an upper yoke and a lower yoke, and the upper yoke and the lower yoke are grounded.
Since the first and second center electrodes and the capacitors are grounded along with the yokes to be shielded, occurrence of spurious can be prevented.
According to a second aspect of the present invention, a high-frequency circuit apparatus includes one of the above-described nonreciprocal circuit devices.
This enables a communication apparatus having low insertion loss and stability in the characteristics to be obtained.